1. Field of the Invention
The present invention relates to a sheet feeding apparatus, and more particularly it relates to a sheet feeding apparatus in which a feeding roller and a conveying roller are driven by a single motor. The sheet feeding apparatus according to the present invention can particularly been used in an image forming apparatus.
2. Related Background Art
In image forming apparatuses of serial type, a carriage on which a recording head is mounted is scanned in a main scanning direction to record an image, and a sheet is conveyed intermittently in a direction (sub-scanning direction) perpendicular to the main scanning direction to record an image on the entire sheet.
With the arrangement as mentioned above, resolution in the main scanning direction is directly determined by resolution of recording means; whereas, connecting portions between the images in the sub-scanning direction are determined by sheet conveying accuracy. That is to say, continuity of the image in the sub-scanning direction is apt to be conspicuous because it is determined by a factor different from that in the main scanning direction, and, it is preferable that conveying means (generally, a conveying roller) is directly connected to a drive motor and is controlled accurately.
On the other hand, in such an apparatus, sheets stacked in a feeding portion are separated and fed by feeding means, and the sheet is subjected to correction of skew-feed by the conveying means (generally, a roller which will be referred to as xe2x80x9cconveying rollerxe2x80x9d hereinafter) and is conveyed to the recording means by the conveying roller. When a roller (referred to as xe2x80x9cfeeding rollerxe2x80x9d hereinafter), is used as the feeding means, the feeding roller must be driven only upon separation and feeding of the sheet and be stopped except this, and, it is preferable that the feeding roller is separated from the sheet when the feeding roller is stopped. Thus, in order to drive the conveying roller and the feeding roller by a single drive motor, drive intermittent means must be provided.
When the sheet is conveyed after the skew-feed of the sheet is corrected, in a state that the feeding roller is always rotated forwardly, the conveying roller must be rotated reversely and then be rotated forwardly. That is to say, even when the drive motor is rotated in either direction, the feeding roller must be designed so that the forward driving force is transmitted to the feeding roller. In a conventional arrangement, a drive transmitting mechanism for forward rotation of the motor and a drive transmitting mechanism for reverse rotation of the motor are switched by using drive switching means, and the numbers of gears used in these drive transmitting mechanisms are differentiated between the drive transmitting mechanisms by one or odd number, and a one-way clutch is incorporated into each of the drive transmitting mechanisms.
In the above-mentioned arrangement, the skew-feed of the sheet is corrected by abutting the sheet by the feeding roller against the reversely rotated conveying roller. Then, the drive motor is rotated forwardly to convey the sheet, thereby entering the sheet into a nip of the conveying roller (nipping). If the sheet is flexed due to an excessive feeding amount of the feeding roller during the correction of the skew-feed, the nipping can be facilitated by an elastic restoring force of the sheet. However, if the sheet has a great rigidity, the skew-feed of the sheet is corrected by the relative slipping movement between the feeding roller and the sheet. Therefore, a force for the nipping becomes only a friction force between the conveying roller and the sheet. Accordingly, in consideration of various kinds of sheets, environment and endurance, only this arrangement cannot ensure the secure nipping and the aid of the feeding roller is required. Thus, the feeding roller is designed so as to continue the forward rotation even after the conveying roller starts the forward rotation.
However, in the above-mentioned arrangement, in a case that a clutch of pendulum type which is relatively inexpensive is used as the one-way clutch, a pendulum movement is caused when the rotational direction of the conveying roller is changed from the reverse rotation to the forward rotation. Therefore, a time lag is created, with the result that the feeding roller will not yet be rotated just when the nipping is needed. Consequently, there arises dispersion in leading end margin and nipping timing differs between the left side and the right side, thereby generating the skew-feed. In order to avoid this, a spring clutch or a one-way clutch of needle type which have less time lag may be used, but these are expensive.
An object of the present invention is to provide a sheet feeding apparatus comprising a feeding roller for separating and feeding a sheet from a sheet stack, a conveying roller for conveying the sheet, a drive motor directly connected to the conveying roller and being rotatable forwardly and reversely, drive intermittent means for selectively transmitting a driving force from the drive motor to the feeding roller, and drive switching means for rotating the feeding roller in a conveying direction regardless of a rotational direction of the drive motor, wherein, in the drive switching means, a spring clutch is used as a one-way clutch for effecting drive transmission when the drive motor rotates the conveying roller in the conveying direction (referred to as xe2x80x9cforward rotationxe2x80x9d hereinafter) and a clutch of pendulum type is used as a one-way clutch for effecting the drive transmission when the drive motor rotates the conveying roller in a direction opposite to the conveying direction (referred to as xe2x80x9creverse rotationxe2x80x9d hereinafter), and a number of gears used for the forward rotation is different from a number of gears used for the reverse rotation by one or odd number.
Another object of the present invention is to provide an image forming apparatus comprising a feeding roller for feeding out a sheet, sheet stacking means for stacking the sheets and movable toward the feeding roller to abut the sheets against the feeding roller in association with rotation of the feeding roller, a conveying roller for conveying the sheet fed by the feeding roller, an image forming portion for forming an image on the sheet conveyed by the conveying roller while moving in a direction perpendicular to a sheet conveying direction, a motor for driving the conveying roller directly, and drive transmitting means including a gear portion directly connected to the feeding roller, clutch means for selectively being engaged with and disengaged from the gear portion thereby to transmit a driving force of the motor to the gear portion, and engagement means for engaging the clutch means with the gear portion in association with the movement of the image forming portion, wherein when the gear portion and the clutch means are once engaged with each other, the drive transmitting means rotates the feeding roller through a predetermined number of revolutions even after the engagement between the gear portion and the clutch means is released, and wherein, after the clutch means are engaged with the gear portion of the feeding roller by the engagement means and before the sheet abuts against the feeding roller, the engagement is released.